Packets transmitted over a link of a packet based network connecting a source to a destination receiver do not necessarily arrive exactly a predetermined time after they are provided for transmission by an application layer of the source. Rather, different packets have different end-to-end transmission times through the network. The difference between the transmission times of different packets from the source to the receiver is referred to as jitter. The receiver generally reorganizes the packets in a reception buffer, which regulates their timing and reorders the packets if necessary.
It is generally desirable to minimize the jitter, so that the jitter buffer of the receiver can be relatively small and in order to reduce the end-to-end delay, which is affected by the largest expected jitter. Reducing the jitter also reduces the processing complexity of the receiver. Furthermore, some network devices cannot operate at high jitter levels.
The overall jitter is formed of two types of jitter: network jitter, which is due to variations in delays along the network, and transmitter jitter, which is due to variations in the packet transmission times at the transmitter itself.
Many methods have been suggested for overcoming network jitter.
U.S. Pat. No. 6,038,230 to Ofek, the disclosure of which is incorporated herein by reference, describes a transmission method in which packets are transmitted between links in predefined periodic intervals.
PCT publication WO03/003630 to Wetzel et al., the disclosure of which is incorporated herein by reference, describes a method of distributing time stamps in order to overcome problems associated with jitter.
U.S. patent publication 2004/0076187 to Peled, the disclosure of which is incorporated herein by reference, describes a method of synchronizing between communication terminals of asynchronous packet networks.
UK patent publication GB 2 359 702 to Humpleman, the disclosure of which is incorporated herein by reference, describes transmission of MPEG video data over a home Ethernet network. To prevent jitter, the MPEG clock of the signals is locked to an internal network clock, so that the receiver can use the internal network clock for timing the signals.
These patents relate to network jitter, but do not deal with the problem of transmitter jitter.
In private networks, the problem of network jitter is less severe, since the owner of the network can limit the use of the network only to packets from a single transmission point.
If a low transmitter jitter is desired, the transmitter needs to accurately time the packets it transmits in order to avoid jitter. This is generally performed by a transmitter with an accurate clock, which clock generates an interrupt which initiates a transmission process each time a packet is to be transmitted. Such frequent interrupts are burdensome on the transmitter, as performing the interrupt requires substantial resources. U.S. Pat. No. 6,907,066 to Nayler, titled “Arrangement for reducing transmitted jitter”, the disclosure of which is incorporated herein by reference, describes one such transmitter.
U.S. Pat. No. 6,292,487 to Kunito et al., titled “Data Transmission System and Method”, the disclosure of which is incorporated herein by reference, describes an ATM data transmission system, in which the time of transmitting packets is adjusted according to the rate at which the data is received by the transmission system.
Such transmitters, however, are very processing intensive, due to the need to perform I/O operations at a very high rate and perform interrupts at a high rate. Therefore, some systems prefer stacking together a block of data that is transmitted together, although this causes jitter of at least the size of the block.